This application claims the benefit of Japanese Patent Application No. 2001-313134, filed Oct. 10, 2001, in the Japanese Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electrophotographic printer, and more particularly, to an apparatus for charging a photosensitive drum in an electrophotographic printer during an initial printing process.
2. Description of the Related Art
A printing procedure of an electrophotographic printer includes charging a surface of a photosensitive drum. In order to improve a quality of a picture printed after the charging of the photosensitive drum, a surface potential of the photosensitive drum needs to be maintained at a predetermined value.
In order to maintain the surface potential of the photosensitive drum at a constant value, a surface potential sensor measuring the surface potential of the photosensitive drum is provided so that an amount of an electrical charge transmitted to the surface of the photosensitive drum can be controlled according to a result of detection of the surface potential sensor. Since the surface potential sensor is usually expensive, the surface potential sensor can be used in only high-grade types of machines.
Accordingly, in the prior art, a charging current used to charge the surface of the photosensitive drum is controlled to maintain the constant value. By controlling the charging current, even when a resistance of a charging roller transmitting the charging current, that is, the electrical charge, toward the surface of the photosensitive drum varies according to a temperature or humidity, the surface potential of the photosensitive drum can be maintained at the constant value.
However, as shown in FIG. 1, if a photosensitive layer corresponding to the surface of the photosensitive drum wears down, a thickness of the photosensitive layer is reduced, and an electrostatic capacity of the photosensitive layer increases. In this case, if the charging current is maintained at the constant value, the surface potential of the photosensitive drum is reduced and thus cannot be maintained at the constant value.
To solve the above and other problems, it is an object of the present invention to provide an electrophotographic printer which can maintain a surface potential of a photosensitive drum at a constant value even when a photosensitive layer corresponding to a surface of the photosensitive drum wears down.
Additional objects and advantageous of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, to achieve the above and other objects, there is provided an electrophotographic printer. The electrophotographic printer includes a charging power supply which generates a variable charging voltage, a charging roller to which the charging voltage generated by the charging power supply is supplied, a photosensitive drum charged by the charging roller, a discharge unit which discharges the charged photosensitive drum, a charging current detection unit which detects a charge value of the charging current flowing between the charging power supply and the charging roller, a transfer power supply which generates a predetermined transfer voltage, a transfer roller to which the transfer voltage generated by the transfer power supply is supplied, a transfer current detection unit which detects a transfer value of transfer current flowing between the transfer power supply and the transfer roller, an arithmetic and control unit which controls the charging voltage generated by the charging power supply, on and off of a rotation of the photosensitive drum, and on and off of the discharge unit and inputs the charging value of the charging current detected by the charging current detection unit and the transfer value of the transfer current detected by the transfer current detection unit, and a memory unit connected to the arithmetic and control unit.
The arithmetic and control unit controls the photosensitive drum to make a plurality of revolutions in a state when the discharge unit is turned off in an initial operation of the electrophotographic printer, controls the charging power supply to generate the charging voltage to obtain the charging current as the surface potential of the photosensitive drum, obtains the transfer current detected by the transfer current detection unit, calculates the charging current needed in obtaining the obtained transfer current in a state when the discharge unit is turned on, and stores the calculated charging voltage in the memory unit, and in an real printing procedure, the arithmetic and control unit rotates the photosensitive drum in a state when the discharge unit is turned on, and equalizes the charging voltage output from the charging power supply to the charge value stored in the memory unit.
According to an aspect of the present invention, the arithmetic and control unit controls the photosensitive drum to make a plurality of revolutions in a state when the discharge unit is turned off in an initial operation of the electrophotographic printer, controls the charging power supply to generate the charging voltage to obtain the charging current as the surface potential of the photosensitive drum, obtains the transfer current detected by the transfer current detection unit, obtains another transfer current at the transfer voltage at several points from the transfer current detection unit in a state when the discharge unit is turned on, calculates the charging current needed in obtaining the obtained transfer current in the state when the discharge unit is turned on, and stores the calculated charging voltage in the memory unit.